The common approach for stress relieving steel in the form of sheet, strip, strapping, wire and the like is to pass the length of steel through a gas-fired or induction furnace which heats the steel as it passes through the furnace to the desired stress relieve temperature. The steel, as it exits from the furnace, is cooled according to various known techniques to achieve the desired properties in the stress relieved steel. Very substantial capital investment is needed to provide a gas-fired furnace of the size which is capable of treating steel sheet and the like. Substantial floor area is needed for such equipment. Another significant problem with a gas-fired furnace is its inability to immediately adjust to changing temperature requirements for stress relieving. Substantial periods of time are needed to bring the furnace up to the desired temperature for stress relieving a particular steel and to adjust that temperature requires additional extended times. Another difficulty with the use of gas-fired furnaces is that the temperature is set for a particular line speed. Should that speed vary, there is thus a change in the temperature to which the steel is elevated. Thus, there are frequent inconsistencies in the characteristics of the gas-fired furnace stress relieved product.
A further requirement in the use of gas-fired furnaces to continuously heat treat steel sheet and the like is to use accumulators. Such a system is disclosed in Canadian Pat. No. 661,066, where although the system is used for making high tensile strap, it demonstrates the use of accumulators in combination with a gas-fired furnace to manufacture strapping. The use of accumulators requires substantial floor area in the plant and also high capital investment in setting up the operation.
In instances where it is desired to stress relieve coiled steel sheet, strip, strapping, wire and the like, instead of uncoiling the steel and passing it through a gas-fired furnace and recoiling it, attempts have been made to heat treat the coil on a bulk basis, which is commonly referred to as "box" stress relieving of coils. Aside from overcoming the internal distortion aspects, an extended period of time is needed to stress relieve the product such as up to three days or more of heat treating. A further drawback with "box" stress relieving is the inconsistencies in the characteristics of the stress relieved steel in the particular coil.
Various attempts have been made to heat metals using infrared radiation. Heat treatment has been on a batch basis where a portion of the metal, such as steel or aluminium, is exposed to infrared radiation to heat the metal. For example, aluminium may be preheated to a desired temperature prior to forming 90.degree. bends in the aluminium by using a break press. Such infrared heaters may be purchased from Barry & Sewell of Minneapolis, Minn.
I have discovered process and apparatus using high intensity infrared heating units which may be adapted to stress relieve steel sheet, strip, strapping, wire and the like for heating it to a stress relieve temperature. A significant problem encountered in developing the use of high intensity infrared radiation for stress relieving steel sheet and the like was that the high power inputs necessary to heat the sheet caused a significant percentage of burn-out of the radiation emitter lamps. This problem has been overcome by controlling the temperature of the lamp electrodes during use in heat treating steel at high linear speeds.
It is, therefore, an object of the invention to stress relieve steel sheet, strip, strapping, wire and the like using high intensity infrared radiation.
It is a feature of the invention to stress relieve such steel in using high intensity shortwave, infrared radiation heaters to obtain precise control on the consistent heating of the steel to the desired stress relieve temperature and the ability to adjust the intensity of the radiation, dependent upon line speed, so that consistent temperatures for stress relieving steel are maintained.
It is, therefore, an advantage of this invention to provide an apparatus which is adapted to stress relieve types of steel where substantially lower capital costs are needed to heat the steel.
It is another advantage of the invention that the line may be started and stopped at will without causing damage to the steel, because the intensity of the infrared radiation may be adjusted almost instantaneously dependent upon detected change in speed of the steel passing through the unit.
It is another advantage of the invention that precise control can always be maintained on heat treating steel and that lighter gauges of steels may be stress relieved using this process.
A further advantage of the invention is an option to eliminate accumulators in view of the start/stop feature of the invention.